This invention relates to a method of spacing a cathode from a grid during electron gun construction and particularly to such method using contact spacing elements.
In an electron gun, the surface of the electron-emissive coating of a cathode is axially positioned relative to a control grid. The cathode and grid are fixed to, and electrically insulated from, one another by glass support rods. The electron gun also includes a cathode support and a screen grid which is spaced from the control grid. When the emissive surface of the cathode is positioned too close to the control grid, arcing between the cathode and the control grid may occur, and the cutoff voltage may change. A very small change in the spacing distance such as 0.001 inch (0.0254 mm), may change the cutoff voltage of the electron gun by about 60 volts.
There are several methods presently in use for establishing cathode-to-grid spacing. In the most common, an air probe is inserted through the grid apertures and the cathode is moved toward the probe until a predetermined backpressure, related to spacing, is measured. Similar methods utilize optical or microscopic measurements to set spacing. Other methods use a spacing element that is positioned in direct contact with the cathode and grid. In these later methods, either a mechanical spacer remains in the gun or the spacer is soluble or volatile and is thereafter removed by dissolution in water or by heat.
Cathode-grid spacings set by permanent spacers can vary from tube to tube due to parts and assembly tolerances. Gas flow gages require the insertion of a nozzle through the grid aperture which can damage that aperture. Optical measurements are too slow for production. Soluble or volatile spacers introduce materials that are difficult to remove completely and can either harm the emissive surface or deposit material on insulators thus causing electrical leakage between gun elements.